Sliding element with coating

Abstract

The invention relates to a sliding element with a base layer and a coating provided on an outer surface of the base layer, wherein a shaped surface is defined such that it represents an outer surface of the coating, and wherein a Radon transform g(θ,ρ) of a square section of this shaped surface is defined, the range of values ​​of which is normalized to values ​​between 0 and 1, and wherein θ represents the angle and ρ the distance to the origin of the coordinate system, and wherein the Radon transform g(θ,ρ) is reduced in the ρ-direction by a factor of 1 / √2, and wherein for each angle θ a standard deviation of the Radon transform σρ(θ) and a value (θ) = Σρ |g(θ,p )- g| the Radon transform is defined, where g is the mean of all g(θ,ρ) of the respective angle θ, and where σmax, σmin, σ , is the maximum standard deviation, the minimum standard deviation and the mean of the standard deviations, and , , is the maximum of the values ​​,the minimum of the values ​​and the mean of the values ​​is , and where a parameter A is defined as A = (σmax-σmin) / σ * () / ().,

Description

[0001] The invention relates to a sliding element with a base layer and a coating provided on an outer surface of the base layer, wherein a shaped surface is defined such that it represents an outer surface of the coating, and wherein a Radon transform g(θ,ρ) of a square segment of this shaped surface is defined, the range of values ​​of which is normalized to values ​​between 0 and 1, and wherein θ represents the angle and p the distance to the origin, and wherein the Radon transform g(θ,ρ) is truncated in the p-direction by a factor of 1 / V², and wherein for each angle θ a standard deviation of the Radon transform σ ρ (θ) and a value (θ) = Σ ρ |g(θ,ρ )- g | is defined by the Radon transform, where g the mean of all g(θ,ρ) of the respective angle θ, and where σmax, σmin, σmin, is the maximum standard deviation, the minimum standard deviation, and the mean of the standard deviations, as well as the maximum of the values , the minimum of the values and the mean of the values is , and where a parameter A is defined as A = (σ max -σ min ) / σ * ( - ) / .

[0002] With hydrodynamic plain bearings, it is important to note that the necessary lubricating film for low-wear operation must first be built up by the rotation of a machine component, such as a shaft. This rotation of the shaft creates a hydrodynamic pressure distribution in the lubricant, which counteracts the external bearing force. When this force is sufficiently large, which occurs when the shaft reaches a minimum rotational speed, the shaft floats onto the lubricant and separates from the mating surface. Four operating ranges can be distinguished for hydrodynamic plain bearings: static friction, boundary friction, mixed friction, and / or fluid friction. In the absence of relative motion (static friction) or at low rotational speeds (boundary friction), the two contact surfaces are in direct contact with each other.As the rotational speed increases, the lubricating film becomes more load-bearing, and the plain bearing enters a transitional range. This is referred to as a mixed friction range. Above the minimum rotational speed lies the range of the plain bearing in which only fluid friction occurs, and therefore there is practically no wear.

[0003] Particularly in modern plain bearing applications in the industrial and energy sectors, plain bearings can be subjected to extreme load cases where the actual load typically exceeds the bearing's nominal load and the sliding speed is very low. These extreme load cases lead to critical mixed, static, and boundary friction conditions with significantly increased wear and a higher probability of failure due to overload-related damage characteristics. An example of this is plain bearings in wind turbines, where the load is typically high and the rotational speed can vary considerably during operation depending on the prevailing wind conditions.

[0004] To reduce the failure probability of plain bearings, additional coatings are frequently used in such bearings to optimize their tribological properties with regard to friction coefficients and wear in the ranges of static friction, boundary friction, and mixed friction. An example of such a coating can be found in EP1764522B1, which shows a bearing element comprising a support layer, a bearing metal layer arranged above the support layer, and a sputtered sliding layer arranged above the bearing metal layer, on which a sliding lacquer layer is applied. GB2363433A, on the other hand, shows a composite sliding material containing a polytetrafluoroethylene resin as its main component, which is intended to improve wear resistance when used as a bearing material.

[0005] This approach is disadvantageous in that it only improves the problem symptomatically, namely by mitigating the negative effects of operating the sliding bearing in an unfavorable hydrodynamic condition.

[0006] A more attractive solution, however, is to improve the hydrodynamic pressure build-up between the sliding bearing and the machine part, as this reduces the operating window, which is defined by the load and the rotational speed of the machine part, in which tribologically unfavorable operating conditions occur (static friction, boundary friction and mixed friction), thus solving the problem at the root cause.

[0007] An interesting approach involves structuring the sliding surface of the sliding element in such a way that it inherently supports hydrodynamic pressure build-up. However, a look at the microscopic surface topography of such sliding surfaces reveals that mechanically machined sliding surfaces are particularly unfavorable for hydrodynamic pressure build-up, as the number of microscopically available hydrodynamic wedge surfaces is reduced.

[0008] Therefore, the object of the present invention is to provide a sliding element with a base layer and a coating provided on an outer surface of the base layer, wherein an outer surface of the coating is shaped in such a way that the hydrodynamic pressure build-up is favored by the presence of a plurality of microscopic, hydrodynamic wedge surfaces, in particular compared to mechanically machined sliding surfaces.

[0009] The problem is solved by a sliding element according to claim 1.

[0010] An advantageous embodiment of the invention is a sliding element with a base layer and a coating provided on an outer surface of the base layer. A shaped surface is defined such that it represents a shape of an outer surface of the coating. Furthermore, a Radon transform g(θ,ρ) of a square segment of this shaped surface is defined, the range of values ​​of which is normalized to values ​​between 0 and 1, where θ is the angle and p is the distance to the origin. The Radon transform g(θ,ρ) is also symmetrically canceled in the p-direction by a factor of 1 / V². For each angle θ, a standard deviation of the Radon transform σρ(θ) and a value are defined. (θ) = Σ ρ | g(θ,ρ)- g̅| of the Radon transform is defined. Here, g is the mean of all g(θ,ρ) of the respective angle θ, and σmax are the maximum standard deviations, σmin the minimum standard deviation, and σmin the mean of the standard deviations, as well as the maximum of the values , the minimum of the values and the mean of the values Furthermore, a parameter A is defined as A = (σ max -σ min ) / σ* ( - ) / The form surface has a parameter A of < 1.0, advantageously < 0.9, and particularly advantageously < 0.7.

[0011] The phrase "Furthermore, the Radon transform g(θ,ρ) is symmetrically shortened in the p-direction by a factor of 1 / V²" means that the Radon transform only considers points g(θ,ρ) whose distance to the origin p < ρmax / √2. Here, ρmax is the maximum distance to the origin. This is necessary to prevent edge effects caused by the square section of the shape surface or the square shape of the original surface topography image.

[0012] This design has the advantage that such a sliding element greatly promotes the hydrodynamic pressure build-up between the sliding element and a supported machine part, simplifying the transition from static friction, boundary friction, and mixed friction states, or reducing the operating time in these states. This is achieved by structuring such a surface to provide a multitude of hydrodynamic wedge surfaces at the microscopic level, particularly in comparison to mechanically machined sliding surfaces.

[0013] In an advantageous embodiment of the invention, the form surface is defined by an orthogonal xyz coordinate system, with the x-axis representing a length dimension of the sliding element, the y-axis representing a width dimension of the sliding element, and the z-axis representing a dimension representative of a thickness dimension of the sliding element. A dimension representative of the thickness dimension of the sliding element can, for example, be a geometric thickness dimension or a gray value of a surface topography image. Other representative dimensions or sizes are also conceivable. In an advantageous embodiment of the invention, the dimension representative of a thickness dimension of the sliding element is a gray value of a surface topography image of the coating of the sliding element of a specific size and bit depth.The procedure for determining parameter A is as follows: In a first step, the surface topography image is corrected for outliers and unmeasured points, and subsequently, a plane that can be described by a second-order polynomial is subtracted for background correction. Before determining the standard deviations σ ρ(θ) and the values... The Radon transform g BG(θ,ρ ) is corrected as follows: A correction image with the same size and bit depth as the surface topography image is generated, where the gray value of each pixel is randomly distributed. A Radon transform of this correction image is then calculated, and the corrected Radon transform g* BG(θ,ρ) is calculated by subtracting the Radon transform of the correction image from the Radon transform g BG(θ,ρ). Finally, the standard deviations σ ρ(θ) and the values ​​are determined. using the corrected Radon transform g* BG(θ,ρ) .

[0014] When determining parameter A from a surface topography image acquired using confocal imaging, the resolution in a plane whose normal is parallel to the normal of the sliding surface must be less than 1 / 5 of the structure created by mechanical processing, particularly grooves. Furthermore, the measurement spot must be sufficiently large to contain at least 10 such structures created by mechanical processing, particularly grooves. The resolution in a direction parallel to a direction normal to the sliding surface must be less than 50 nm. If the structures created by mechanical processing have been completely or partially masked by a coating, the specifications must still be met with regard to the structures beneath the coating.

[0015] In an advantageous embodiment of the invention, the outer surface of the base layer has a parameter A > 1 and exhibits at least an arithmetic mean roughness Ra of 1 µm. A surface with a parameter A > 1 is disadvantageous with regard to hydrodynamic pressure build-up, since the surface is designed in such a way that the number of microscopic, hydrodynamic wedge surfaces is reduced compared to a surface with a parameter A < 1. An example of a surface with A > 1 is, for instance, the surface of a turned or drilled component. In a possible embodiment of the invention, the outer surface of the base layer with A > 1 is therefore covered by a coating with a surface having a parameter A of < 1.0, advantageously < 0.9, and particularly advantageously < 0.7.This has the advantage that, in addition to other tribologically favorable properties of the coating, the hydrodynamic pressure build-up of the sliding element is significantly improved.

[0016] In an advantageous embodiment of the invention, the parameter of < 1.0, advantageously < 0.9, and particularly advantageously < 0.7, is achieved solely through microscopic unevenness on the outer surface of the coating, and the coating exhibits a macroscopically uniform thickness. Macroscopically uniform thickness means that the coating is of uniform thickness, taking into account microscopic unevenness or roughness on the outer surface of the coating or the base layer, and any process tolerances during the application of the coating. The parameter is therefore influenced exclusively by microscopic unevenness on the outer surface of the coating and not by intentional, macroscopic changes in the thickness of the coating.

[0017] In an advantageous embodiment of the invention, an outer surface of the coating has an arithmetic mean roughness value Ra according to DIN EN ISO 4287, selected from a range with a lower limit of 0.1 µm and an upper limit of 15 µm. This has the advantage that such a roughness profile, in combination with a surface with parameter A < 1.0, provides, on the one hand, a large number of microscopic, hydrodynamic wedge surfaces as well as microscopic oil reservoirs of favorable size, and, on the other hand, allows tribologically unfavorable operating conditions to be exited particularly quickly.

[0018] In an advantageous embodiment of the invention, the coating is designed as a polymer-based layer, in particular as a polyimide-, polyamide-, polyamide-imide-, polyetherketone-, or polyhaloolefin-based layer, especially PTFE, or a mixture thereof. The base layer consists of a material selected from the group comprising aluminum-based alloys, copper and copper-based alloys, bismuth-based alloys, tin and tin-based alloys, silver and silver-based alloys, and zinc-based alloys.

[0019] In particular, polymer coatings with imide groups show an improvement in terms of wear resistance and a reduction in the tendency to chip. Polyetherketone- or polyhaloolefin-based coatings, such as PTFE, have the advantage of exhibiting particularly good lubricity in low-speed applications.

[0020] To produce the polymer-based layer, a dispersion is created from the individual components—a polymer precursor, possible solid lubricant particles, possible metal oxide particles, and optionally possible hard particles—using a dispersing agent, in particular an organic solvent or solvent mixture, according to standard procedures. This dispersion is then applied to the surface to be coated using methods common in paint technology, such as spraying, brushing, or dipping.

[0021] For the formation of polymer-based layers with a parameter A < 1.0, the solvent or solvent mixture used and the temperature of the substrate or the surface to be coated are particularly crucial. Using PAI-based layers as an example, suitable solvent mixtures would include the following solvents: NMP, NEP, NBP, GBL, DSMO, or DMF, mixed with one or more of the following highly volatile blending solvents: xylene, cyclohexane, n-hexane, white spirit, naphtha, propanol, n-butanol, isobutanol, or butyl glycol. Favorable mixing ratios between the solvent and the blending solvents would be a solvent content between 70 and 90% by mass and a blending solvent content of 10 to 30% by mass. In addition to the parameter A, the resulting surface roughness can also be influenced by the mixing ratio.Furthermore, a suitable substrate temperature for PAI-based layers lies between 70 °C and 80 °C. According to the invention, the substrate would be the base layer onto which the coating is applied.

[0022] In a further advantageous embodiment of the invention, the coating is applied via pulsed laser deposition or suspension plasma spraying. The base layer consists of a material selected from the group comprising aluminum-based alloys, copper and copper-based alloys, bismuth-based alloys, tin and tin-based alloys, silver and silver-based alloys, and zinc-based alloys. Both manufacturing processes are particularly suitable for producing surface areas with a parameter A of < 1.0. Suspension plasma spraying has the additional advantage that the application of the powder as a suspension enables the deposition of finer microstructures, which can improve the tribological properties of the coating.Both methods can be used in particular to produce coatings with fine columnar microstructures, which can influence the stress distribution in the area of ​​the tribological contact and the deformation of the coating under heavy load.

[0023] In an advantageous embodiment of the invention, the sliding element is a sliding element in a wind turbine gearbox designed as a planetary gearbox. Improving the hydrodynamic pressure build-up in such sliding bearings is particularly important because the load occurring in this type of bearing is typically high, and the rotational speed can vary considerably during operation depending on the prevailing wind conditions.

[0024] In an advantageous embodiment of the invention, the sliding element achieves good tribological properties under critical mixing, adhesion, and boundary friction conditions. The sliding element exhibits a dynamic coefficient of friction < 0.04, advantageously < 0.03, and particularly advantageously < 0.02. Furthermore, in a tribological test to determine the wear height, the wear height is a maximum of 50 µm, advantageously a maximum of 20 µm, and particularly advantageously a maximum of 10 µm.

[0025] By definition, a ring-on-disc test with low sliding speed and high load is performed to determine the dynamic coefficient of friction according to the critical mixing, static, and boundary friction conditions. The test program is structured as follows: Starting with an initial run-in phase at a sliding speed of 0.2 m / s and a load (expressed as surface pressure in MPa) of 20 MPa for 7200 seconds (2 hours), after which the sliding speed is linearly reduced at a rate of 0.025 m / s² to 0.02 m / s, and the load is continuously increased over 2800 s (at a rate of approximately 0.0179 MPa / s) until a load of 70 MPa is reached, at which point the load remains constant at this value. This test range (sliding speed 0.02 m / s at a load of 70 MPa) is maintained for 7200 seconds, thus determining the mean value of the dynamic coefficient of friction over the last 1800 seconds.The test temperature is 60 °C, and Amsoil Power Transmission EP Gear Lube ISO-320 is used as the lubricant. The following tribometer was used for the tests: WAZAU tribometer TYPE TRM 1000. By definition, a ring-on-disc test with low sliding speed and high load is performed to determine the wear height according to the critical mixing, adhesion, and boundary friction conditions. The following test program is used: Starting from an initial sliding speed of 0 m / s, the sliding speed is increased linearly by 1 / 20 m / s until a sliding speed of 1 m / s is reached. This speed is held for 20 seconds, and then the sliding speed is reduced linearly by 1 / 20 m / s until it reaches 0 m / s. The sliding speed of 0 m / s is held for 20 seconds before the test cycle begins again identically.Throughout the entire test cycle, the load is maintained at a constant 10 MPa. The test cycle is repeated a total of 2000 times, corresponding to a sliding distance of 90,000 m. The test temperature is 60 °C, and Amsoil Power Transmission EP Gear Lube ISO-320 is used as the lubricant. The following tribometer was used for conducting the tests: WAZAU Tribometer TYPE TRM 1000. The wear height was subsequently determined optically using a Leica DCM8 optical 3D surface measurement system. The wear height is determined at four locations on the specimen, each offset by 90°. Two area measurements are taken at each location: one measurement area is in the region of maximum wear (the "wear trench"), and the other is located outside the wear trench. The height differences between the areas are then averaged.Such tribological tests are familiar to experts, and the characteristic values ​​are usually determined on various samples.

[0026] According to a particular embodiment, a method for manufacturing a sliding element is provided comprising the following process steps: Providing a sliding element with a mechanically machined base layer; applying a coating to an outer surface of the base layer to produce a sliding element according to claim 1.

[0027] The invention is explained below in a non-limiting manner schematically with reference to preferred embodiments.

[0028] They show: Fig. 1 a sliding element in half-shell shape and in oblique view. Fig. 2 a sliding element in a planetary gearbox of a wind turbine. Fig. 3an evaluation of parameter A for sliding elements with a coating according to the invention in comparison with sliding elements with a mechanically machined sliding surface.

[0029] In Fig. 1 Figure 1 shows a variant embodiment of a sliding element 1 in oblique view. The sliding element 1 consists of a support body 2, a base layer 3, and a coating 4. The sliding element 1 according to Fig. 1The sliding element 1 has the form of a half-shell. This can be combined with another half-shell to form the sliding bearing. The two half-shells can have the same or different constructions. It is also possible for the sliding element 1 to be designed in the form of a third-shell, etc. In this case, the sliding element 1 is combined with the corresponding number of additional sliding elements to form the sliding bearing. The support body 2 consists of a metallic material, usually steel, but can also be made of a material that can perform the same or a similar function, namely providing the mechanical strength of the sliding element 1. For example, various copper alloys, such as brass or bronze, can also be used. Within the scope of the invention, direct coatings of components, such as the eye of a connecting rod, are also possible.In this case, the support body 2 is formed by the respective component itself. The base layer 3 is formed by a bearing metal alloy. Such bearing metal alloys are known from the prior art. For example, the bearing metal alloy can consist of a material selected from a group including aluminum-based alloys, copper and copper-based alloys, bismuth-based alloys, tin and tin-based alloys, silver and silver-based alloys, and zinc-based alloys. Although in . Fig. 1Although the sliding element 1 is depicted as a three-layer bearing element, it can also have fewer or more than three layers. For example, the coating 4 can be applied directly to the support body 2; in this case, the support body would constitute the base layer according to the invention. Likewise, conventional intermediate layers, such as at least one bonding layer or at least one diffusion barrier layer, can be arranged as required. This at least one bonding layer can be arranged between the support body 2 and the base layer 3 and / or between the base layer 3 and the coating 4. The at least one diffusion barrier layer can be arranged between the support body 2 and the base layer 3 and / or between the base layer 3 and the coating 4.The coating 4 can be designed as a polymer-based layer with solid lubricant particles and / or metal oxide particles, but can also be implemented as metallic, ceramic, or composite coatings. Suitable solid lubricants for this application are already well known from the prior art.

[0030] Figure 2Figure 5 shows the gearbox 5 of a wind turbine, which has a sun gear 6 that is non-rotatably connected to a shaft 7 leading to the generator rotor or another gear stage (not shown). The sun gear 6 is surrounded by several planet gears 8, for example two, preferably up to nine. Both the sun gear 6 and the planet gears 8 have teeth 9, 10 that mesh with each other. The planet gears 8 are mounted on an axle 11, the so-called planet axle, formed by a planet pin. These axles 11 can either be formed integrally with at least part of a planet carrier 12 or they are inserted as separate components in bores of the planet carrier 12. A ring gear 13 is arranged above the planet gears 8, which also has at least partial teeth 14 on an inner surface that mesh with the teeth 10 of the planet gears 8.The ring gear 13 is non-rotatably connected to a rotor shaft 15 of the wind turbine rotor. The teeth 9, 10, 14 are designed as helical gears.

[0031] One possible embodiment for arranging the sliding element 16 is as a bearing bushing that is rotationally fixed to a planet gear 8, for example by means of an interference fit or another suitable method. However, another possible embodiment is that the sliding element 16 is arranged directly on the shaft 11, in the area of ​​the planet gear 8 bearing, and / or directly on the planet gear 8 itself, in the area of ​​the bore receiving the planet pin. Other arrangements of the sliding element known to those skilled in the art are, of course, also conceivable.

[0032] Figure 3Figure 3 shows an evaluation of parameter A in the form of a box plot. Graph 3.1 represents a sliding element with a coating according to the invention, whose parameter A < 1.0, thus promoting hydrodynamic pressure build-up through the presence of a multitude of microscopic, hydrodynamic wedge surfaces. Graph 3.2, on the other hand, represents a series of measurements from mechanically machined sliding surfaces without a coating according to the invention, with a parameter A > 1.0. The mechanical machining involved, for example, grinding, turning, or fine boring.

Claims

1. Sliding element with a base layer and a coating arranged on an outer surface of the base layer, wherein a shaped surface is defined such that it represents an outer surface of the coating, and wherein a Radon transform g(θ,ρ) of a square segment of this shaped surface is defined, the range of values ​​of which is normalized to values ​​between 0 and 1, and wherein θ is the angle and p is the distance to the origin, and wherein the Radon transform g(θ,ρ) is symmetrically canceled in the p-direction by a factor of 1 / V², and wherein for each angle θ a standard deviation of the Radon transform σ ρ (θ) and a value (θ) = Σ ρ |g(θ,ρ )- g | is defined by the Radon transform, where g the mean value of all g(θ,ρ) of the respective angle θ, and where σ max , σ min, σ , the maximum standard deviation, the minimum standard deviation and the mean of the standard deviations, as well as the maximum of the values , the minimum of the values and the mean of the values is, where a parameter A is defined as A = (σ max -σ min ) / σ * ( - ) / and wherein the form surface has a parameter A of < 1.0, advantageously < 0.9, particularly advantageously of < 0.

7.

2. Sliding element according to claim 1, characterized by the fact that The shape surface is defined via an orthogonal xyz coordinate system, where the x-axis represents a length dimension of the sliding element, the y-axis represents a width dimension of the sliding element, and the z-axis represents a dimension representative of a thickness dimension of the sliding element.

3. Sliding element according to claim 2, characterized by the fact thatThe dimension representative of a thickness dimension of the sliding element consists of gray values ​​of a surface topography image of the coating of the sliding element of a certain size and a certain bit depth.

4. Sliding element according to one of the preceding claims, characterized by the fact that the outer surface of the base layer has a parameter A > 1 and the outer surface of the base layer has at least an arithmetic mean roughness Ra of 1 µm.

5. Sliding element according to one of the preceding claims, characterized by the fact that the parameter A of the mold surface of < 1.0, advantageously < 0.9, particularly advantageously of < 0.7, is achieved only by microscopic unevenness on the outer surface of the coating and the coating has a macroscopically uniform thickness.

6. Sliding element according to one of the preceding claims, characterized by the fact thatan outer surface of the coating has an arithmetic mean roughness value Ra according to DIN EN ISO 4287, selected from a range with a lower limit of 0.1 µm and an upper limit of 15 µm.

7. Sliding element according to one of the preceding claims, characterized by the fact that the coating is designed as a polymer-based layer, in particular as a polyimide, polyamide, polyamide-imide, polyetherketone, or polyhalogenolefin-based layer, for example PTFE, or a mixture thereof, and wherein the base layer consists of a material selected from a group comprising aluminium-based alloys, copper and copper-based alloys, bismuth-based alloys, tin and tin-based alloys, silver and silver-based alloys, and zinc-based alloys.

8. Sliding element according to one of claims 1-6, wherein the coating is applied via pulsed laser deposition or suspension plasma spraying and wherein the base layer consists of a material selected from the group comprising aluminium-based alloys, copper and copper-based alloys, bismuth-based alloys, tin and tin-based alloys, silver and silver-based alloys, zinc-based alloys.

9. Sliding element according to one of the preceding claims, characterized by the fact that It is a sliding element in a wind turbine gearbox, which is designed as a planetary gearbox.

10. Sliding elements according to one of the preceding claims, characterized by the fact thatThe sliding element achieves good tribological properties under critical mixing, adhesion and boundary friction conditions, for which the sliding element has a dynamic coefficient of friction < 0.04, advantageously < 0.03, particularly advantageously < 0.02, and in a tribological test to determine the wear height, the wear height is a maximum of 50 µm, advantageously a maximum of 20 µm, particularly advantageously a maximum of 10 µm.

11. Method for manufacturing a sliding element comprising the process steps of: - providing a sliding element with a mechanically machined base layer - applying a coating to an outer surface of the base layer to manufacture a sliding element according to claim 1.

Images